Respiratory diseases of pigs can be classified into two broad categories based on the extent and duration of overt disease: those that affect large numbers of pigs and may be serious but of limited duration, and those that persist in a large number of pigs for indefinite periods. Diseases in the first category can be costly, but the losses are limited rather than ongoing. They include swine influenza, classical swine fever, the pneumonic forms of pseudorabies, porcine circovirus-associated disease, and porcine reproductive and respiratory syndrome (PRRS). The causal viruses may persist in a herd, but outbreaks of overt disease tend to be self-limiting.
The most important syndromes in the second category are mycoplasmal pneumonia and pleuropneumonia. Atrophic rhinitis, once considered to be an important cause of respiratory disease in swine, has declined substantially as a result of eradication programs. Haemophilus parasuis infections may be a problem in some herds, particularly those infected with PRRS virus. Moderate levels of atrophic rhinitis caused by Bordetella bronchiseptica alone may not be too consequential but, when coupled with toxigenic strains of Pasteurella spp infection, are an important cause of economic loss due to decreased rate of growth and reduced feed conversion in young pigs. Enzootic pneumonia, when caused by mycoplasma alone, is of little consequence; however, when it is combined with secondary infection, eg, Pasteurella multocida, the resulting condition may be severe. Actinobacillus pleuropneumoniae may be associated with considerable losses in some herds.
The severity and economic importance of diseases in the second category also are related to population density and to the type and size of a herd. Although mortality usually is low, economic damage results from an adverse and uneven effect on growth rate, decreased feed efficiency, and additional costs of drugs. Viral co-infections, such as highly virulent strains of PRRS, can considerably increase deaths and costs. Finally, it must be stressed that respiratory disease problems in pigs are most commonly the result of multiple agents (co-infection) versus a single pathogen.
It is possible to set up herds free of diseases in the second category by techniques such as specific pathogen free repopulation or segregated early weaning, or by buying pigs from a pneumonia-free herd. The latter method is the least expensive, but because the etiology of diseases in the second category is complex, all the pigs should be purchased from one source.
In the absence of air filtration, it is difficult to keep herds free of respiratory diseases. Aerosols have been suspected as sources of pathogen entry onto naive farms. Organisms such as Mycoplasma hyopneumoniae have been proven to be transported up to 9.2 km via aerosols, depending on climate, terrain, and density of pigs in the locality.
Multiple site production or an all-in/all-out policy, in which the entire barn or air space is emptied before refilling, can very effectively minimize the potential effect of chronic pneumonia.
